1. Field of the Invention
The present invention relates to the monitoring and/or analysis of fluids located in a well. More particularly, the invention relates to apparatus and methods utilizing optical fluid analysis for in situ monitoring of downhole fluids over long periods of time.
2. State of the Art
Those skilled in the art will appreciate that the ability to conduct an analysis of formation fluids downhole (in situ) is extremely desirable. With that in mind, the assignee of this application has provided a commercially successful borehole tool, the MDT (a trademark of Schlumberger) which extracts and analyzes a flow stream of fluid from a formation in a manner substantially as set forth in co-owned U.S. Pat. Nos. 3,859,851 and 3,780,575 to Urbanbsky which are hereby incorporated by reference herein in their entireties. The OFA (a trademark of Schlumberger), which is a module of the MDT, determines the identity of the fluids in the MDT flow stream and quantifies the oil and water content based on the previously incorporated related patents. In particular, U.S. Pat. No. 4,994,671 to Safinya et al., which is hereby incorporated by reference herein in its entirety, provides a borehole apparatus which includes a testing chamber, means for directing a sample of fluid into the chamber, a light source preferably emitting near infrared (NIR) rays and visible light, a spectral detector, a data base means, and a processing means. Fluids drawn from the formation into the testing chamber are analyzed by directing the light at the fluids, detecting the spectrum of the transmitted and/or backscattered light, and processing the information accordingly (and preferably based on the information in the data base relating to different spectra), in order to quantify the amount of water and oil in the fluid. As set forth in previously incorporated U.S. Pat. No. 5,266,800 to Mullins, by monitoring optical absorption spectrum of the fluid samples obtained over time, a determination can be made as to when a formation oil is being obtained as opposed to a mud filtrate. Thus, the formation oil can be properly analyzed and quantified by type. Further, as set forth in U.S. Pat. No. 5,331,156 to Hines et al., which is hereby incorporated by reference in its entirety herein, by making optical density measurements of the fluid stream at certain predetermined energies, oil and water fractions of a two-phase fluid stream may be quantified.
While the Safinya et al., Mullins, and Hines et al. patents represent great advances in downhole fluid analysis, and are particularly useful in the analysis of oils and water present in the formation, they do not address in detail the gases which may be plentiful in the formation. The issue of in situ gas quantification is addressed in U.S. Pat. Nos. 5,167,149 to Mullins et al., and 5,201,220 to Mullins et al., and in O. C. Mullins et al., xe2x80x9cEffects of high pressure on the optical detection of gas by index-of-refraction methodsxe2x80x9d, Applied Optics, Vol. 33, No. 34, pp. 7963-7970 (Dec. 1, 1994) all of which are hereby incorporated by reference herein in their entireties, where a rough estimate of the quantity of gas present in the flow stream can be obtained by providing a gas detection module having a detector array which detects light rays having certain angles of incidence. While rough estimates of gas quantities are helpful, it will be appreciated that more accurate measurements are often necessary.
Co-owned U.S. Pat. No. 4,994,671, which is hereby incorporated by reference in its entirety herein, discloses an apparatus and method for analyzing the composition of formation fluids through the use of spectroscopy. Spectroscopy has been used downhole for distinguishing between oil and water (in the near infrared spectrum), and for distinguishing among oils (in the visible spectrum). However, for several reasons, downhole spectroscopy has not been suggested for distinguishing between gas and oil or for distinguishing among different hydrocarbon gases such as methane (CH4), ethane (having methyl components (CH3)), and higher hydrocarbons which contain predominantly methylene (CH2). First, because the density of a gas is a function of pressure, and because downhole pressures can vary by a factor of thirty or more, the dynamic range of the gas densities likely to be encountered downhole is extremely large. As a result, it is believed that the dynamic range of the spectral absorption at frequencies of interest is also extremely large such as to make a measurement unfeasible; i.e., the sensitivity of the downhole spectroscopy equipment is typically incapable of handling the large dynamic ranges that are expected to be encountered. Second, due to fact that the condensed phase of hydrocarbon (oil) has a much higher density at downhole pressures than the gas phase, it is believed that a thin film of liquid oil on the OFA window can yield significant absorption. Thus, where an oil film was present, interpretation of the results would yield a determination of a rich gas mixture, where no or little amount of hydrocarbon gas was actually present. Third, the type of spectral analysis typically done uphole to distinguish among hydrocarbon gases cannot be done downhole. In particular, in uphole applications, individual gas constituents are detected by modulating a narrow band source on and off of mid-infrared absorption lines of the gas, where a resulting oscillation in absorption at each modulation frequency would indicate a positive detection of a particular gas. However, at the high pressures encountered downhole, not only are the narrow gas absorption spectral lines merged, but mid-infrared spectroscopy is hindered by the extreme magnitude of the absorption features. Fourth, spectrometers are typically sensitive to changes in temperature, and elevated temperatures encountered downhole can induce spectral changes of the gas sample, thereby complicating any data base utilized.
Co-owned U.S. Pat. No. 5,859,430, which is hereby incorporated by reference in its entirety herein, discloses a method and apparatus for the downhole compositional analysis of formation gases which utilizes a flow diverter and spectrographic analysis. More particularly, the apparatus includes diverter means for diverting formation gas into a separate stream, and a separate gas analysis module for analyzing the formation gas in that stream. By providing a diverter means and a separate gas analysis module, the likelihood of a having a thin film of oil on the cell window is decreased substantially, thereby improving analysis results. Also, by providing one or more cells with different path lengths, issues of dynamic range are obviated, because where the pressure is higher, light will not be fully absorbed in the cell having a short path length, whereas where the pressure is lower, there will be some absorption in the cell having the longer path length. The methods and apparatus of the ""647 application are useful in determining what types of gas are present in the formation fluid, but are not particularly useful in determining other important measurements such as the gas-oil ratio (GOR).
The gas-oil ratio is a particularly important measurement for newly discovered oil. The GOR is conventionally defined as the volume of gas at STP (standard temperature and pressure) in cubic feet divided by the number of stock tank barrels of oil in a quantity of formation fluid. A GOR of 6,000 ft3/bbl represents approximately equal mass fractional amounts of gas and oil. The GOR must be known in order to establish the size and type of production facilities required for processing the newly discovered oil. For example, a very large GOR of approximately 11,000 ft3/bbl will require the construction of expensive gas handling facilities. It is therefore important to make an accurate measurement of GOR in newly discovered oil so that the appropriate financial investment in production facilities is made.
Co-owned U.S. Pat. No. 5,939,717 to Mullins, which is hereby incorporated by reference in its entirety herein, discloses apparatus and methods for determining in situ a GOR in a geological formation. In particular, using the borehole tool apparatus disclosed in co-owned U.S. Pat. No. 5,859,430, formation fluid is subjected to near infrared (NIR) illumination, and the NIR absorption spectrum is detected downhole. By comparing the NIR absorption(s) at one or more wavelengths associated with gas (e.g., 1.667 microns) to the NIR absorption(s) associated with oil (1.720 microns), a GOR determination can be made.
The methods and apparatus of U.S. Pat. No. 5,939,717 have been particularly useful and commercially successful in making in situ GOR determinations for newly discovered oil.
Recently, however, there has been an emphasis in the art on providing permanent sensors in producing oil wells, rather than running tools on a regular basis through the well, thereby disrupting production. Thus, as set forth in PCT Publication WO 98/50681 to Johnson et al., systems are provided for providing permanent-type fiber optic-based. sensors at various locations along a producing well in order to obtain temperature, pressure, and fluid flow measurements which can be used to make decisions in controlling production. As broadly taught in the Johnson et al. PCT publication, a light source may be provided uphole or downhole to inject light into a fluid sample. Where the light source is uphole, the light is carried by fiber optics to the fluid sample, and light which has interacted with the fluid sample is returned by fiber optics to a spectrometer uphole for measurement.
While the Johnson et al. PCT publication discloses numerous uses of permanent-type fiber optic-based sensors located along a producing well, the disclosure is schematic in nature, and details for implementation of such uses are not set forth. Thus, various issues are not dealt with which make reasonable implementation difficult or impossible, or which could cause the system to fail partially or completely over time. In addition, certain applications, such as an accurate measurement of the GOR at various locations along the producing well are not disclosed or suggested in the Johnson et al. PCT publication.
It should be appreciated by those skilled in the art that it would be advantageous to be able to simultaneously obtain GOR determinations along various locations in a well via the use of permanent sensors. By having numerous sensors at different locations along the well, it is possible to determine whether certain zones of the well are producing gas while others are producing oil, as well as the GOR ratio of zones producing both gas and oil. By providing permanent sensors, these determinations can be made over a long period of time, and certain zones can be appropriately controlled in order to optimize production.
It is therefore an object of the present invention to provide permanent optical sensor systems for making GOR determinations along the length of a producing well.
It is another object of the invention to provide permanent fiber optic systems in wellbores which provide robust measurements over long periods of time.
In accord with the objects of the invention which will be discussed in more detail below, systems for measuring the gas-oil ratio of fluid being produced in a wellbore are provided and generally include an uphole light source which produces a high amplitude near infrared signal at selected wavelengths, an uphole spectrometer, a processor coupled to the spectrometer for making GOR determinations, a plurality of optical cells in contact with wellbore fluid and located along various locations of the wellbore, and a fiber optic system which couples the uphole light source to the xe2x80x9cinput sidexe2x80x9d of each of the optical cells, and which couples the xe2x80x9coutput sidexe2x80x9d of each of the optical cells to the spectrometer. Preferably, the light source produces high amplitude NIR light at or around 1.72 microns (an oil peak), 1.67 microns (a methane peak), 1.6 microns (a baseline), and 1.58 microns (a carbon dioxide peak).
Various embodiments of the fiber optic system are provided. A first system is a dual path system where a measurement path includes a fiber which carries source light to the optical cell and a fiber which carries light from the optical cell to the spectrometer, and a reference fiber which parallels the measurement path but which does not send light through the optical cell. A second system is a shared path system, where a first fiber carries light from the light source to the optical cell, a second fiber carries light from the optical cell to the spectrometer, and a splitter and optical coupler near the optical cell are used in the first fiber and second fiber paths respectively in conjunction with a reference/delay line (third) fiber. A third system is a main fiber line system which utilizes a single fiber to carry light from the source to the optical cell and from the optical cell to the spectrometer, and a splitter near the optical cell to carry light to and from the optical cell, and another splitter and optical coupler in conjunction with a reference/delay line (second fiber). A fourth system is a complimentary or redundant system similar to the shared path system, but utilizing two additional splitters near the optical cell. In the complimentary system, if either of the two fibers carrying light from the light source to the cell or from the cell to the spectrometer must be taken out of service, the system will still work.
Any of the four optical systems can be utilized for each of the downhole cells by repeating the system for each cell. Alternatively, provided the light source is powerful enough, additional splitters can be utilized in conjunction with additional delay lines so that multiple cells can use a single system.
According to another aspect of the invention, the downhole optical cell can take various configurations. According to one preferred embodiment, the optical cell includes a floating piston which pulls in and pushes out sample fluid. The floating piston is preferably fit in the cell such that when it moves, it wipes the optical window of the cell in order to keep the window clean. According to another embodiment, the optical cell is a probe type cell such as disclosed in one or more of the previously incorporated patents. The probe type cell may have fixed length or variable length paths. Regardless, ultrasonic window cleaners are preferably utilized in the probe type cell to prevent buildup of oil or deposits.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.